1. Field of the Invention
The present invention is directed to a chromium compound-containing catalyst for polymerizing olefins, particularly alpha-olefins, also known as 1-olefins, comprising at least two different chromium species and to a method of polymerizing such olefins in the presence of the catalyst of this invention. More particularly, the invention is directed to a method of producing polymers of ethylene or copolymers of ethylene and at least one C.sub.3 -C.sub.10 alpha-olefin, which are useful in the fabrication of high strength films.
2. Description of Related Art
Chromium compound-containing alpha-olefin polymerization catalysts, also known as Phillips catalysts, have been extensively described in the literature. They are formed by supporting chromium or a compound of chromium, e.g., chromium trioxide, or a compound calcinable thereto, on a refractory oxide support material, for example, silica, alumina, zirconia, thoria or silica-alumina, and heating the oxide support material in a non-reducing atmosphere, preferably an oxidizing atmosphere, to produce an active polymerization catalyst. The produced catalyst is used to polymerize olefins, usually 1-olefins, in a suitable process, such as the so-called "solution form" or "particle form" process. In the "solution form" process, the monomeric olefin, which is normally ethylene or a mixture of ethylene with up to about 40 wt. % of other 1-olefins, is contacted with a suspension of the catalyst in a liquid hydrocarbon which is a solvent for the polymer at the polymerization temperature employed. In the "particle form" process, the monomer olefin is contacted with a suspension or a fluidized bed of the catalyst particles in a fluid medium under conditions such that the polymeric olefin forms as solid particles suspended in or fluidized in the fluid medium. The fluid medium can be, for example, a liquid hydrocarbon or a gas. Examples of suitable liquid hydrocarbons are isobutane and n-pentane. Examples of suitable gases are nitrogen or argon mixed with the gaseous monomer, or the undiluted gaseous monomer. Processes of this type are described in, for example, U.K. published patent specifications 790,195; 704,641; 853,414; 886,784 and 899,156. It is also known to modify such catalysts with a titanium compound, for example, to render the catalyst capable of producing polyolefins having increased melt index (i.e., lower average molecular weight) or to increase the stress crack resistance of the produced polyolefin. Catalysts of this type are described in, for example, U.S. Pat. No. 3,622,521 to Hogan et al and U.K. published patent specifications 1,334,662 and 1,326,167.
U.S. Pat. No. 3,351,623 to Walker et al discloses a catalyst for polymerizing ethylene at a temperature in the range of 275.degree. to 335.degree. F. under solution form process conditions, the catalyst being one which forms on mixing (1) an oxide component selected from the group consisting of silica; mixtures of silica and alumina containing up to 25 wt. % alumina; and mixtures of chromium oxide and at least one material selected from the group consisting of silica, alumina, zirconia and thoria, at least part of the chromium oxide being in the hexavalent state at the initial contacting of the monomer therewith, (2) an organo metallic component of formula R.sub.x M where R is selected from the group consisting of hydrogen and alkyl, aryl, cycloalkyl, alkoxy radicals and combinations of these radicals containing up to 12 carbon atoms, at least one R group being a hydrocarbon radical; M is selected from the group consisting of aluminum, gallium, indium, thallium, zinc, boron, lead, lithium, zirconium, cobalt, magnesium and tin; and x is an integer equal to the valence of M, and (3) a vanadium component selected from the group consisting of vanadium chelates and vanadyl chelates. Walker et al seek to produce a catalyst that permits the use of high polymerization temperatures to obtain relatively low melt index polymers.
Downs, U.S. Pat. No. 4,368,302, discloses a modified alpha-olefin catalyst composition used in preparing polymers of ethylene or copolymers of ethylene and higher alpha-olefins, having high average molecular weight and a relatively broad molecular weight distribution, as evidenced by relatively high values of melt flow ratio (MFR), referred to by Downs as melt index ratio (MIR). The catalyst composition of Downs is prepared by supporting chromium oxide on a refractory oxide support material, adding at least one tetravalent titanium compound and heating to activate the product. The monomer is contacted with the active polymerization catalyst in the presence of one or more organometallic compounds, e.g., dibutylmagnesium or triethylaluminum. The amount of the organometallic compounds is 0.1-100% by weight (wt %), preferably 1 to 10 wt %. The refractory oxide support has a mean particle diameter of about 20-150 microns (.mu.).
U.K. Patent Application 2,023,153 to Eve discloses an ethylene polymerization catalyst comprising: (A) a heat activated supported chromium oxide catalyst, and (B) a catalyst modifier comprising a magnesium compound, such as a dialkyl magnesium, preferably dibutyl magnesium, and a commercially available product believed to comprise a mixture of dibutyl magnesium, dialkyl magnesium and butyl-alkyl magnesium. The polymerization process, conducted in the presence of this catalyst, produces polyethylene or ethylene and higher alpha-olefin copolymers having broad molecular weight distribution and low melt index. The quantity of the catalyst modifier is such that the atomic ratio of magnesium to chromium in the modified catalyst system is 0.01:1 to 100:1, preferably, 0.01:1 to 10:1.
Stricklen et al., U.S. Pat. No. 4,374,234, disclose a silica-supported chromium catalyst to which is added up to 5 ppm of an aluminumalkyl or dihydrocarbomagnesium compound, e.g., dibutylmagnesium, in order to reduce the induction period and increase catalyst activity. The addition of the aluminum or the magnesium compounds has only a modest effect on polymer properties.
Most of the previously-known Phillips catalyst compositions were used to produce high density, medium and low molecular weight products. It is desirable, however, to have a Phillips' catalyst composition capable of producing high molecular weight, high density olefin polymers which are used to manufacture superior strength films, considered to be premium commercial products.